The field of the disclosure relates generally to fiber communication networks, and more particularly, to optical networks utilizing wavelength division multiplexing.
Telecommunications networks include an access network through which end user subscribers connect to a service provider. Bandwidth requirements for delivering high-speed data and video services through the access network are rapidly increasing to meet growing consumer demands. At present, data delivery over the access network is growing by gigabits (Gb)/second for residential subscribers, and by multi-Gb/s for business subscribers. Present access networks are based on passive optical network (PON) access technologies, which have become the dominant system architecture to meet the growing high capacity demand from end users.
Gigabit PON and Ethernet PON architectures are conventionally known, and presently provide about 2.5 Gb/s data rates for downstream transmission and 1.25 Gb/s for upstream transmission (half of the downstream rate). 10 Gb/s PON (XG-PON or IEEE 10G-EPON) has begun to be implemented for high-bandwidth applications, and a 40 Gb/s PON scheme, which is based on time and wavelength division multiplexing (TWDM and WDM) has recently been standardized. A growing need therefore exists to develop higher/faster data rates per-subscriber to meet future bandwidth demand, and also increase the coverage for services and applications, but while also minimizing the capital expenditures (CAPEX) and operational expenditures (OPEX) necessary to deliver higher capacity and performance access networks.
One known solution to increase the capacity of a PON is the use of WDM technology to send a dedicated wavelength signal to end users. Current detection scheme WDM technology, however, is limited by its low receiver sensitivity, and also by the few options available to upgrade and scale the technology, particularly with regard to use in conjunction with the lower-quality legacy fiber environment. The legacy fiber environment requires operators to squeeze more capacity out of the existing fiber infrastructure to avoid costs associated with having to retrench new fiber installment. Conventional access networks typically include six fibers per node, servicing as many as 500 end users, such as home subscribers. Conventional nodes cannot be split further and do not typically contain spare (unused) fibers, and thus there is a need to utilize the limited fiber availability in a more efficient and cost-effective manner.
Coherent technology has been proposed as one solution to increase both receiver sensitivity and overall capacity for WDM-PON optical access networks, in both brown and green field deployments. Coherent technology offers superior receiver sensitivity and extended power budget, and high frequency selectivity that provides closely-spaced dense or ultra-dense WDM without the need for narrow band optical filters. Moreover, a multi-dimensional recovered signal experienced by coherent technology provides additional benefits to compensate for linear transmission impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), and to efficiently utilize spectral resources to benefit future network upgrades through the use of multi-level advanced modulation formats. Long distance transmission using coherent technology, however, requires elaborate post-processing, including signal equalizations and carrier recovery, to adjust for impairments experienced along the transmission pathway, thereby presenting significant challenges by significantly increasing system complexity.
Coherent technology in longhaul optical systems typically requires significant use of high quality discrete photonic and electronic components, such as digital-to-analog converters (DAC), analog-to-digital converters (ADC), and digital signal processing (DSP) circuitry such as an application-specific integrated circuit (ASIC) utilizing CMOS technology, to compensate for noise, frequency drift, and other factors affecting the transmitted channel signals over the long distance optical transmission. Coherent pluggable modules for metro solution have gone through C Form-factor pluggable (CFP) to CFP2 and future CFP4 via multi-source agreement (MSA) standardization to reduce their footprint, to lower costs, and also to lower power dissipation. However, these modules still require significant engineering complexity, expense, size, and power to operate, and therefore have not been efficient or practical to implement in access applications.
Furthermore, according to Nielsen's Law, if current trends continue, high-end end users are expected to require as much as 10 Gb/s by 2023, and 100 Gb/s by 2029. For the new and upcoming generations of communication systems performing under these requirements, the data speed will also need to be matched for upstream communications. However, conventional PONs seeking to approach the 100 Gb/s aggregating data rate suffer from several limitations, due to the reliance on traditional direct detection techniques, that render 100 Gb/s technically and economically infeasible for these PONs. The conventional direct detection PONs, for example, are known to have poor receiver sensitivity, to experience power fading due to chromatic dispersion at high symbol rates and long transmission distances, and to utilize bandwidth- and power-inefficient modulation. Besides frequency selectivity and linear detection, Coherent for PONs demonstrates superior receiver sensitivity, which can be translated to extend reach and split ratio.
In the downlink of conventional PONs, the complexity limits on the transceiver in an optical line terminal (OLT) at the headend, central office, and/or hub are less stringent than the limits placed on a receiver in an optical network unit (ONU), since the cost of the OLT transceiver, which sends and receives data to and from multiple ONUs, is shared by all end users supported in the respective network. In contrast, the cost of each ONU is born solely by the respective end user. Accordingly, lower costs and lower complexities will more significantly impact the ONU than the OLT. For this reason, the complexity and high cost of conventional coherent transceivers has been limited to point to point (P2P) applications, but prevented from implementation in point to multipoint (P2MP) PON applications. That is, despite the significant advantages offered by digital coherent technology, the complexity and high cost of conventional coherent transceivers has not been economically feasible for individual ONUs at the home location of each subscriber end-user.
P2P and P2MP applications differ in that they P2P connection provides a link between one transmitter and one receiver, whereas a P2MP application provides a link between one transmitter and multiple receivers. Accordingly, in the coherent paradigm, only two coherent transceivers may be needed in a P2P link, whereas the number of coherent transceivers needed in the P2MP link (i.e., one coherent transceiver for each ONU) may be significant (as many as 500, in the example above).
Therefore, the laser source is of critical importance for the realization of such coherent optical transmission systems. That is, one type of laser may not simply be substituted for another type without significantly affecting the network. Additionally, the frequency and phase noise of the laser will also significantly affect the performance of conventional optical coherent transceivers, and impairments therefrom have to be mitigated by carrier-phase recovery (CPR) techniques, since frequency and phase noise are directly related to each other, and are closely related to the linewidth of the laser.
Furthermore, the modulation speed and transmission distance of the network also will strongly depend on the spectral linewidth of the laser. That is, narrower linewidths are required for higher modulation speeds (data rates) and longer distance transmissions. Some conventional coherent transceivers use an external cavity laser (ECL). From the performance perspective, ECLs have demonstrated superior performance capabilities for coherent systems, sufficient for present long haul and metro distance sensitivity requirements. However, within the access environment, ECLs are considered prohibitively expensive if deployed at each ONU at an end user's home location. In contrast, Fabry-Perot laser diodes (FP-LD) and weak-resonant-cavity laser diode (WRC-FPLD) based transmitters are considerably less expensive than the costly externally tunable lasers such as ECLs or distributed feedback (DFB)/distributed Bragg reflector (DBR) lasers. However, use of these relatively lower-cost, simpler FP lasers is limited by transmission bandwidth and capacity, particularly in direct-detection systems, and is not applicable for coherent systems in the conventional use form.